U.S. patent application number 11/431297 was filed with the patent office on 2007-11-15 for thermal oxidation protective surface for steel pistons.
Invention is credited to Miguel Azevedo, Warran Lineton.
Application Number | 20070261663 11/431297 |
Document ID | / |
Family ID | 38683952 |
Filed Date | 2007-11-15 |
United States Patent
Application |
20070261663 |
Kind Code |
A1 |
Lineton; Warran ; et
al. |
November 15, 2007 |
Thermal oxidation protective surface for steel pistons
Abstract
A piston (120) and method for making a piston (120) for a
fuel-injected diesel engine adapted to withstand the damaging
effects of fuel injection plume-induced oxidation in the regions of
the piston bowl (134) and rim (130). The surfaces of the piston
crown (126) targeted by the fuel injection plume (138) are first
coated with a corrosion-resistant and oxidation-resistant
composition applied as a slurry or by a thermal spraying technique,
such as HVOF or plasma spraying. Thereafter, a high energy
industrial laser beam irradiates the as-sprayed coating to increase
its density, while simultaneously reforming its microstructure so
as to fuse, alloy, and materially bond the coating material with
the underlying steel substrate, thereby resulting in a durable
protective surface for the steel piston crown (126).
Inventors: |
Lineton; Warran; (Ann Arbor,
MI) ; Azevedo; Miguel; (Ann Arbor, MI) |
Correspondence
Address: |
DICKINSON WRIGHT PLLC
38525 WOODWARD AVENUE
SUITE 2000
BLOOMFIELD HILLS
MI
48304-2970
US
|
Family ID: |
38683952 |
Appl. No.: |
11/431297 |
Filed: |
May 10, 2006 |
Current U.S.
Class: |
123/270 ;
123/193.6; 123/276; 123/668; 29/888.04 |
Current CPC
Class: |
B23K 26/34 20130101;
C23C 4/18 20130101; Y10T 29/49249 20150115; B23K 2103/50 20180801;
Y02T 10/125 20130101; F02F 3/14 20130101; F02B 23/0603 20130101;
B23K 35/38 20130101; F02B 75/08 20130101; F02F 3/26 20130101; F02F
3/12 20130101; F02B 23/0696 20130101; C23C 24/04 20130101; Y02T
10/12 20130101; F16J 1/01 20130101; B23K 2101/003 20180801; B23K
26/32 20130101; C23C 4/01 20160101 |
Class at
Publication: |
123/270 ;
123/193.6; 123/276; 123/668; 029/888.04 |
International
Class: |
F02B 19/00 20060101
F02B019/00; F02F 3/00 20060101 F02F003/00; F02F 3/26 20060101
F02F003/26; F02B 75/08 20060101 F02B075/08; B23P 15/10 20060101
B23P015/10 |
Claims
1. A method for improving the corrosion resistance of a piston
crown for an internal combustion engine, said method comprising the
steps of: providing a piston having a crown presenting an exterior
crown surface; preparing a coating material consisting essentially
of a corrosion-resistant and oxidation-resistant composition;
applying the coating material to the piston crown such that the
coating material adheres to the crown surface having an as-applied
microstructure and an as-applied porosity less than 100% full
material density; and irradiating the coating with a high energy
laser beam to increase the density of the coating while
simultaneously reforming the microstructure and creating a material
bond between the coating and the crown surface.
2. The method of claim 1 further including masking a portion of the
coating to prevent irradiation from the laser beam.
3. The method of claim 2 wherein said masking step includes
temporarily covering a portion of the crown with a reflective
metallic shield.
4. The method of claim 3 wherein the piston crown includes a
generally annular rim and a concave combustion bowl set below the
rim, said step of temporarily covering a portion of the crown
including covering one of the rim and the bowl but not the other of
the rim and the bowl.
5. The method of claim 4 wherein the piston crown includes at least
one valve pocket formed into the rim, said step of temporarily
covering a portion of the crown including covering one of the rim
and the valve pocket but not the other of the rim and the valve
pocket.
6. The method of claim 1 wherein said step of applying the coating
material includes forcibly propelling the spray material toward the
piston crown into a gaseous flow generated by a combustion
process.
7. The method of claim 6 wherein said step of placing the spray
material into a gaseous flow includes forcing the gaseous flow
through an accelerator nozzle.
8. The method of claim 1 wherein said step of forcibly propelling
the spray material includes producing a DC electric arc.
9. The method of claim 8 wherein said step of producing a DC
electric arc includes ionizing an inert gas to produce a
high-temperature plasma jet.
10. The method of claim 1 wherein said step of irradiating the
coating includes employing a high-power direct diode laser.
11. The method of claim 1 wherein said step of forcibly propelling
the spray material includes applying the spray material to less
than all of the exterior crown surface.
12. The method of claim 1 wherein said step of providing a piston
includes forming the piston from a material composition including a
steel alloy.
13. The method of claim 1 wherein the exterior crown surface has a
plume contact zone comprising that portion of the exterior crown
surface to be subsequently targeted by a fuel injection plume from
about 5.degree. BTDC to about 10.degree. ATDC of piston movement
within a cylinder, and said step of forcibly propelling the spray
material includes the step of coating the plume contact zone of the
exterior crown surface but not the entire exterior crown
surface.
14. A method for operating a steel piston in a fuel-injected diesel
engine, said method comprising the steps of: providing an engine
cylinder having a cylinder head; providing a piston having a crown
including a generally annular rim and a concave bowl set below the
rim, the interface between the rim and the bowl forming a generally
annular lip; reciprocating the piston in the cylinder toward and
away from the cylinder head; forcibly discharging liquid fuel into
the cylinder and toward the lip of the piston crown; combusting the
fuel adjacent the lip of the piston crown; and said step of
providing a piston including altering the surface composition of
the lip of the piston crown by applying a coating material
consisting essentially of a corrosion-resistant and
oxidation-resistant composition to the lip having an as-applied
microstructure and an as-applied porosity less than 100% full
material density, and irradiating the coating with a high energy
laser beam to increase the density of the coating while
simultaneously reforming the microstructure and creating a material
bond between the coating and the lip.
15. The method of claim 14 wherein said step of providing a piston
further includes altering the surface composition of the rim of the
piston crown by forcibly propelling a spray material consisting
essentially of a corrosion-resistant and oxidation-resistant
composition toward the rim such that the particles of spray
material plastically deform upon impact with the rim, and wherein
the spray material adheres to the rim as a durable coating having
an as-sprayed microstructure and an as-sprayed porosity less than
100% full material density, and irradiating the coating with a high
energy laser beam to increase the density of the coating while
simultaneously reforming the microstructure and creating a material
bond between the coating and the rim.
16. The method of claim 15 wherein said step of providing a piston
further includes altering the surface composition of at least a
portion of the bowl of the piston crown by forcibly propelling a
spray material consisting essentially of a corrosion-resistant and
oxidation-resistant composition toward the bowl such that the
particles of spray material plastically deform upon impact with the
bowl, and wherein the spray material adheres to the rim as a
durable coating having an as-sprayed microstructure and an
as-sprayed porosity less than 100% full material density, and
irradiating the coating with a high energy laser beam to increase
the density of the coating while simultaneously reforming the
microstructure and creating a material bond between the coating and
the bowl.
17. A piston for a fuel-injected diesel engine, said piston
comprising: a generally cylindrical skirt having a pair of opposing
pin bores formed transversely therein; a crown affixed atop said
skirt, said crown including a generally annular rim and a concave
bowl set below said rim, and a generally annular lip along the
interface between said rim and said bowl; and said lip having a
bonded surface treatment consisting essentially of an applied
corrosion-resistant and oxidation-resistant composition irradiated
with a high energy laser beam.
18. The piston of claim 17 wherein said piston crown is fabricated
from a base material composition consisting essentially of
steel.
19. The piston of claim 17 wherein said rim has a bonded surface
treatment consisting essentially of a sprayed-on
corrosion-resistant and oxidation-resistant composition irradiated
with a high energy laser beam.
20. The piston of claim 19 wherein at least a portion of said bowl
has a bonded surface treatment consisting essentially of a
sprayed-on corrosion-resistant and oxidation-resistant composition
irradiated with a high energy laser beam.
21. The piston of claim 17 wherein said sprayed-on
corrosion-resistant and oxidation-resistant composition of said
bonded surface treatment is selected from the group consisting of:
Amdry 99C, Inconel 718, Stellite 6, Nickel-Chromium, Chromium, and
alloys thereof.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The subject invention relates to a piston for a diesel
engine and method of making such a piston having a crown specially
treated to resist thermal oxidation degradation and, more
particularly, to such a piston of the steel type used in
fuel-injected diesel engine applications.
[0003] 2. Related Art
[0004] A diesel engine is a reciprocating-piston engine operating
on the well-known thermodynamic cycle in which air is compressed,
fuel is injected into the compressed charge, the auto-combusting
mixture expanded to do work on the piston, and the products
exhausted at completion of the cycle. In large steel pistons such
as used in diesel truck applications, it is common to utilize a
multiple-orifice nozzle to inject fuel during the combustion
process. The nozzle with multiple orifices is located as centrally
as possible above the piston crown and discharges fuel in a radial
spray pattern. A depressed bowl in the piston crown is designed to
ensure that the air-fuel mixture formed from the injection spray
and the rotating air during injection completely fills the
combustion space for optimal performance. If the air-fuel mixture
fails to completely fill the bowl in the piston crown, both air
utilization and power output will suffer. As a result, there will
be a substantial decrease in the anti-polluting emission
characteristics. Likewise, if there is an overlap and the mixture
extends beyond the space between the individual injection events,
the resulting excessive local fuel concentration will lead to air
deficiencies and increased soot formation, again, decreasing the
anti-polluting emission characteristics of the engine.
[0005] In addition to these timing issues, another problem
contributes to a loss of the anti-polluting emission
characteristics designed for the piston. Because the fuel injected
into a diesel engine ignites spontaneously, high Cetane Number
fuels are required. The burning fuel plumes generate intense heat.
The bowl formed in the crown of the steel piston typically
experiences oxidation in areas in close vicinity to and/or on the
top edge of the combustion bowl, i.e., the lip-like interface
between the bowl and the flat top rim of the piston crown. The
result is a plume of radially extending torches extending from the
multiple-orifice nozzle. This torching effect oxidizes the steel up
to Fe.sub.2O.sub.3 status, and the resultant oxides have no
adherence to the underlying, unaffected steel substrate. Mechanical
expansion/contraction processes eventually dislodge this oxidized
layer in a "flaking" manner. Over time, an eroded area can be seen
with the naked eye. This change in the shape of the bowl lip causes
disturbances in the combustion process and contributes to a loss of
the anti-polluting emission characteristics designed into the
combustion bowl of the piston crown. Besides, the eroded areas
weaken the piston structurally. Piston flexing, expansion, and
contraction may induce radial cracks which propagate and could
eventually lead to piston functional failure.
[0006] Various attempts have been proposed to address the issue of
bowl lip oxidation resulting from the intense heat release by
combusting fuel. For example, some have proposed to fabricate the
entire piston crown from a specially formulated alloy designed to
combat oxidation and corrosion. However, the piston crown in a
large diameter piston for truck applications requires a significant
amount of material. Such specially formulated alloys would
significantly increase the cost of a diesel piston.
[0007] Other prior art attempts to address this issue include U.S.
Pat. No. 5,958,332 to Hoeg, granted Sep. 28, 1998. In this example,
a special plate fabricated from a high temperature, corrosion
resistant alloy is welded to the critical areas of a piston or
other engine component. However, the loose-piece fabrication of a
special alloy plate significantly increases the cost of the piston
assembly, as well as adding numerous handling and assembly steps to
the fabrication process. In examples, proposals have been made to
shrink-fit an annulus of high temperature resistant steel or even a
ceramic-based material into the combustion bowl. However, the same
restrictions outlined before apply. Accordingly, there is a
long-felt and as yet unsolved need to address the issue of piston
crown degradation in low-cost steel pistons resulting from
oxidation and the intense heat released by diesel engine combustion
in close proximity to the combustion bowl lip. A commercially
practical solution must be convenient to implement without
increasing the overall product or manufacturing costs, while
retaining long term piston emission compliance performance.
SUMMARY OF THE INVENTION
[0008] According to the subject invention, a method for improving
the oxidation and corrosion resistance of a piston crown for an
internal combustion engine is provided. The method comprises the
steps of providing a piston having a crown presenting an exterior
crown surface; preparing a coating material consisting essentially
of a corrosion-resistant and oxidation-resistant composition;
applying the coating material to the piston crown such that the
coating material adheres to the crown surface having an as-applied
microstructure and an as-applied porosity less than 100% full
material density. The method further includes the step of
irradiating the coating with a high energy laser beam to increase
the density of the coating while simultaneously reforming the
microstructure and creating a material bond between the coating and
the crown surface. The irradiating step actually alloys the coating
and the material of the crown surface, thereby generating a
composite material of properties varying from both that of the
original coating and crown surface material.
[0009] According to yet another aspect of the subject invention, a
method for operating a steel piston in a fuel-injected diesel
engine is provided. The method comprises the steps of providing an
engine cylinder having a cylinder head; providing a piston having a
crown including a generally annular rim and a concave bowl set
below the rim, the interface between the rim and the bowl forming a
generally annular lip; reciprocating the piston in the cylinder
toward and away from the cylinder head; forcibly discharging liquid
fuel into the cylinder and toward the lip of the piston crown; and
combusting the fuel adjacent the lip of the piston crown. The step
of providing a piston includes altering the surface composition of
the lip of the piston crown by applying a coating material
consisting essentially of a corrosion-resistant and
oxidation-resistant composition to the lip having an as-applied
microstructure and an as-applied porosity less than 100% full
material density, and then irradiating the applied coating with a
high energy laser beam to increase the density of the coating while
simultaneously reforming its microstructure and creating a material
bond between the coating and the lip.
[0010] According to yet another aspect of the subject invention, a
piston for a fuel-injected diesel engine is provided. The piston
comprises a generally cylindrical skirt having a pair of opposing
pin bores formed transversely therein. A crown is affixed atop the
skirt. The crown includes a generally annular rim and a concave
bowl set below the rim. A generally annular lip is established
along the interface between the rim and the bowl. The lip has a
bonded surface treatment consisting essentially of an applied,
corrosion-resistant and oxidation-resistant composition irradiated
with a high energy laser beam.
[0011] The subject method and piston structure overcome the
shortcomings and deficiencies of prior art pistons by intentionally
preparing and treating the lip region of the piston crown,
comprising the interface between the annular rim and the concave
bowl so that it can better withstand the abusive effects of
combusting liquid fuel injected from an injector nozzle toward the
lip. A relatively low-cost steel piston made and operated according
to the subject invention can achieve substantially longer service
life and is capable of maintaining long term piston emission
compliance performance, as well as piston structural integrity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] These and other features and advantages of the present
invention will become more readily appreciated when considered in
connection with the following detailed description and appended
drawings, wherein:
[0013] FIG. 1 is a perspective view of a prior art steel piston for
a diesel engine;
[0014] FIG. 2 is a top view of a prior art piston as in FIG. 1 and
depicting a centrally located multiple-orifice fuel injection
nozzle with a plurality of radially extending fuel injection
sprays;
[0015] FIG. 3 is a fragmentary cross-sectional view taken generally
along Lines 3-3 in FIG. 2;
[0016] FIG. 4 is a fragmentary cross-section of a steel diesel
engine piston according to the subject invention illustrating the
process of forcibly propelling a spray material of
corrosion-resistant and oxidation-resistant composition onto the
most vulnerable portions of the piston crown;
[0017] FIG. 5 is a cross-sectional view as in FIG. 4, but showing
all of the vulnerable areas of the piston crown having been coated
with the durable coating material;
[0018] FIG. 6 is a fragmentary cross-sectional view of a piston
crown as in FIG. 5, but depicting the surfaces after they have been
irradiated with a high energy laser beam to increase the density of
the coating while simultaneously reforming its microstructure and
creating a material bond between the coating and the vulnerable
areas of the piston crown;
[0019] FIG. 7 is perspective view of a piston according to the
subject invention and including a first annular mask applied over
the rim section of the piston crown to prevent unwanted irradiation
on certain portions of the coated surface;
[0020] FIG. 8 is a fragmentary cross-sectional view taken generally
along Lines 8-8 in FIG. 7 depicting a high energy laser beam
irradiating the coating in the bowl region of the piston crown upon
which the first mask is applied;
[0021] FIG. 9 is a perspective view of the piston including a
second mask covering the bowl region and the valve pockets so as to
expose only the flat, upper rim section of the piston crown;
[0022] FIG. 10 is a fragmentary cross-sectional view taken
generally along Lines 10-10 in FIG. 9 and depicting a laser beam
reflecting off of the second mask so as not to irradiate the valve
pockets while in the process of irradiating the coating on the rim
section;
[0023] FIG. 11 is a perspective view of the piston showing a third
mask applied to the top of the piston crown and exposing only the
valve pockets so that the coating in this region can be irradiated
by the high energy laser beam; and
[0024] FIG. 12 is a fragmentary cross-sectional view taken
generally along Lines 12-12 in FIG. 11 and depicting the high
energy laser beam irradiating the coating in the region of the
valve pocket to complete the surface preparation of the crown of
the piston.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0025] Referring to the figures, wherein like numerals indicate
like or corresponding parts throughout the several views, a steel
piston for a fuel-injected diesel engine is generally shown at 20
in FIG. 1. The piston 20 is of the type adapted for use in a
fuel-injected diesel engine. The piston 20 comprises a generally
cylindrical skirt portion 22, having a pair of opposing pin bores
24 formed transversely therein. The skirt 22 guides and supports
the piston 20 as it reciprocates in the cylinder (not shown) of a
diesel engine, while the pin bores 24 receive a wrist or gudgeon
pin, which attaches to the upper end of a connecting rod (not
shown) and, ultimately, to the crank shaft of the engine. A crown,
generally indicated at 26, is affixed atop the skirt 22. In the
preferred embodiment of this invention, the skirt 22 and crown 26
are integrally formed of a unitary steel material. The composition
of material can be selected from any of the known varieties,
including but not limited to the relatively low-cost SAE 4140 H.
Instead of the depicted single-piece design, the piston 20 could be
of the so-called "articulating" type, wherein the crown 26 can
pivot slightly relative to the skirt 20 through a common connection
about the wrist pin.
[0026] The crown 26 includes a plurality of ring grooves 28 to
receive compression and/or oil rings (not shown). The ring grooves
28 are formed into the cylindrical outer, sliding surface of the
crown 26 which, at its upper end, intersects a crown rim 30. The
rim 30 is a generally flat, annular region comprising the
uppermost, top portion of the piston 20. Commonly, although not
necessarily, one or more valve pockets 32 are formed into the rim
30 to provide clearance space for the exhaust and/or intake valve
heads 25 (shown in phantom in FIG. 3) when the piston 20 is in its
top dead center ("TDC") position.
[0027] The center inner region of the crown 26, bounded by the rim
30, is known as the bowl, and is generally indicated at 34. The
bowl 34 comprises a combustion-chamber segment formed as a cavity
in the top of the piston crown 26. A multi-orifice nozzle 36,
centered above this extended combustion recess in the crown 26,
injects the fuel in a plurality of radial jets or plumes 38. The
configuration of the nozzle 36 and its projected fuel plume 38
utilizes the depressed and swirling configuration of the bowl 34,
combined with the energy in the injected fuel stream to optimize
the space in which the air and fuel interact and combustion of the
fuel develops. The bowl 34 may include a peaked or domed center,
which falls away toward a recessed trough 42. The trough 42 is a
generally annular feature whose upper, ascending face rejoins the
rim 30. The interface between the rim 30 and the ascending face of
the trough 42 comprises a generally annular lip 44, which may or
may not slightly overhang the trough 42 as shown in FIG. 3.
[0028] In the average diesel engine, the plume of injected fuel 38
initiates at about 5.degree. before top dead center ("BTDC") and
continues until about 10.degree. after top dead center ("ATDC") of
piston movement. As such, the plume 38, whose trajectory remains
generally constant, strafes a surface area of the piston crown 26
which can be characterized as the plume contact zone. The plume
contact zone is, therefore, that portion of the exposed crown
surface that is targeted by the fuel injection plume 38 from about
5.degree. BTDC to about 10.degree. ATDC of piston movement within
the cylinder, including the upper, ascending surface of the trough
42, the lip of 44, and the rim 30, together with the valve pockets
32. The plume contact zone generally does not include the entire
exposed surface area of the bowl 34. Because of the intense heat
released by the combustion of the fuel in close proximity to the
plume contact zone, the steel composition of the piston crown 26 in
a prior art piston has a tendency to oxidize up to Fe.sub.2O.sub.3
status. The oxides which result from the transformation no longer
adhere to the substrate steel material and are rapidly dislodged as
flakes through expansion and contraction processes. Over time but
well inside what should otherwise be the useful life of an engine,
the expanding eroded areas significantly deteriorate the
anti-polluting emission characteristics designed into the
combustion bowl 34 of the piston 20. Structural integrity can, with
time, be severely compromised as well. As perhaps best shown in
FIG. 2, these eroded areas will be most pronounced in those regions
of the lip 44 coinciding with the spray plumes 38.
[0029] Referring now to FIGS. 4-12, an improved piston and method
for making and operating an improved piston according to the
subject invention is shown. For convenience, reference numbers
corresponding to those set forth above are applied to corresponding
features of the piston, but with the prefix "1" distinguishing the
subject invention from the prior art.
[0030] The subject invention is directed toward a piston 120 having
a crown 126 whose surface is modified and enhanced in the plume
contact zone so as to better withstand the intense heat released by
diesel engine combustion in close vicinity to the surface of the
crown 126. FIGS. 4 and 5 depict a fragmentary cross-section of the
piston crown 26 as it is sprayed or otherwise treated with a
corrosion-resistant and oxidation-resistant composition, such as
Amdry 995C, Inconel 718, Stellite 6, nickel-chromium, chromium, or
a mixture of these compositions. The corrosion-resistant
composition can be applied as a paste-like slurry. Preferably,
however, the coating process is carried out as a thermal spray
process of either the combustion type or the electric arc type.
Such processes have been known by the descriptive term
"metalizing." Combustion-type thermal spray processes may include,
but are not limited to, powder flame spray, wire/rod flame spray,
detonation spray, and high velocity oxygen fuel ("HVOF") spray.
Electric arc processes include, but are limited to, arc wire spray
and plasma spray.
[0031] Using the HVOF spray process as an example, a pressurized
chamber gun 146 uses the combustion of acetylene, hydrogen,
propane, propylene, or the like to produce a hot, high-pressure
flame. The flame is forced through a DeLaval nozzle to accelerate
the carrier gas to supersonic velocities. Feed stock powder can be
fed axially into the high-pressure combustion chamber 146 or
directly through the side of the nozzle. Feed stock powders may be
selected from the group of materials as set forth above. While HVOF
is not the only thermal spray process capable of applying a
satisfactory coating of corrosion-resistant and oxidation-resistant
composition over the vulnerable surfaces of the piston crown 126,
it is nevertheless an acceptable example of the variety of spray
processes which can be used.
[0032] In addition to the traditional thermal spraying processes
described above, it is also possible and included within the
intended scope of this invention to utilize a so-called "cold
spray" thermal spraying process. According to a cold spray
technique, small particles in the 1-50 micron size are accelerated
to supersonic velocities and applied to the surface of a work part.
In one configuration, helium or nitrogen is injected at high
pressure into a pressurized chamber and heated to
300.degree.-700.degree. C. Powder feed stock, such as one of the
above-described corrosion-resistant and oxidation-resistant
compositions, is introduced into the gas stream, which is not hot
enough to melt the particles. The solid powder/gas mixture is then
passed through a DeLaval nozzle, where the particles are
accelerated to supersonic velocities. The particles impact the
substrate with enough kinetic energy to produce a mechanical bond
without melting and/or solidification, however, it does not produce
a metallurgical bond.
[0033] FIG. 5 represents the piston crown 126 whose plume contact
zone is fully coated with the spray material, forming a durable
coating having an as-sprayed microstructure and as-sprayed porosity
less than 100% full material density. In other words, the
composition of the spray material, after it is fully applied to the
relevant surface area of the crown 126, possesses a characteristic
microstructure and a material density which is less than 100% fully
dense. Those portions of the bowl 134 which are outside of the
plume contact zone are not coated with the corrosion-resistant,
oxidation-resistant material. To complete the transition of this
piston crown 126 to the modified robust, long-life piston crown 126
according to the subject invention, a high energy laser beam is
then used to irradiate the spray material coating causing a fusion
of the coating material, together with the underlying steel
substrate of the piston crown 126. One example of such a laser beam
is generated by a so-called High-Power Direct Diode Laser ("HPDL").
The two materials (coating and substrate) intermix under the
influence of the laser beam and alloy themselves so as to increase
the density of the coating above that of its as-sprayed condition
(FIG. 5), while simultaneously reforming the microstructure of the
coating. Furthermore, a material bond between the coating and the
crown substrate is established by the subsequent irradiation
process. The resulting modified piston crown 126 is depicted in
FIG. 6.
[0034] Because the surface geometry of a piston crown 126 over its
plume contact zone is complex, numerous passes and orientations of
the high energy laser beam are required to fully and evenly
irradiate the as-sprayed coating. In order to prevent certain areas
of as-sprayed coating from being irradiated at an unintended angle
of incidence relative to the laser beam, it is preferred to mask
certain portions of the coating both prior to and subsequent to the
irradiation process.
[0035] Turning now to FIGS. 7 and 8, a first mask is generally
indicated at 148. The first mask temporarily covers the coating in
the areas of the rim 30 and valve pockets 32, while exposing the
coating applied in the region of the trough 142. Thus, while the
first mask 148 is in place, a high energy laser 150 is free to
irradiate that portion of the trough 142 to which the coating
material has been applied (i.e., in the plume contact zone). As
depicted, the region of the trough 142 below the focal point of the
laser 150 is shown as having been reformed by the fusing qualities
of the laser beam so that the coating material is now alloyed and
materially bonded to the substrate steel material. In this example,
the laser 150 is moving its beam in an upwardly ascending path so
that as it progresses toward the lip 144, all of the coating
material applied to the trough 142 will eventually be fully
irradiated and reformed into a material bond with the steel piston
crown 126. In the preferred embodiment, however, a laser 150 is
selected from the type having a rectangular beam of approximately
12 mm.times.0.5 mm at focus. The long axis of the beam encompasses
the trough 142 to the lip 144 in a vertical direction, but is fixed
in this vertical position. The beam and piston move relative to
each other around the circumference, such as by fixing the laser
and rotating the piston, so that the long axis of the beam sweeps
out the treated area. The resulting fused coating substantially
enhances the physical characteristics of the crown 126, thereby
avoiding the problem of oxidation and permitting the continued use
of a standard piston steel material, such as SAE 4140 H.
Accordingly, the subject invention represents a lower cost solution
than other prior art attempts to overcome the problems associated
with rim and bowl oxidation.
[0036] As the laser 150 traverses its application area within the
bowl 134, the piston 120 may be rotated and/or the laser 150 may be
rotated so that the entire annular region of coated surface area is
adequately irradiated. Should the focal point of the laser 150
extend above the trough 142, it will contact the first mask 148 and
be reflected harmlessly away from the piston crown 126. This is
because the first mask 148 is made from a reflective and thermally
conductive metallic material, such as polished copper.
[0037] Once the trough 142 region has been adequately irradiated,
attention can be directed toward the rim 130, which has also been
coated by the spray material. However, because the valve pockets
132 are depressed below the surface of the rim 130, the focal point
of the laser 150 may not be optimized to effectively irradiate the
coating in the region of the valve pockets 132 at the same time as
the rim 130. Therefore, a second mask 152 may be used, as shown in
FIG. 9. The second mask 152 may also be made from the reflective
metallic shield-like material, such as polished copper, and be
structured so as to cover the valve pockets 132. The only exposed
portions of the spray coated piston crown 126 thereby comprise the
rim surface 130. With the unintended areas effectively shielded by
the second mask 152, the laser 150 can be repositioned to irradiate
the coated surface of the rim 130 as shown in FIG. 10. Again, the
piston 120 and/or the laser 150 may be rotated to cover the entire
rim 130 surface in an efficient manner. Whenever the beam from the
laser 150 strikes the second mask 152, as shown in FIG. 10, it is
reflected harmlessly away from the crown 126, and any absorbed heat
is quickly dissipated. By this method, the valve pockets 132 are
protected from being irradiated by the laser 150 at a non-ideal
setting. The second mask 152 also helps to avoid excessive melting
in the corners and along the edges of the valve pockets 132.
Furthermore, the trough portion 142 is protected from subsequent
attack by the laser 150, which might otherwise result in unintended
metallurgical reformation of the already irradiated surfaces.
[0038] To complete the full irradiation of the as-sprayed coating
on the piston crown 126, a third mask 154 is applied to the top of
the piston crown 126 after the second mask 152 has been removed. As
depicted in FIG. 11, the third mask 154 is designed to cover the
already irradiated rim portion 130 and to leave open the area of
the valve pockets 132. Thus, the position of the laser 150 can be
reset so that its focal point is gauged to the depth of the valve
pockets 132, as shown in FIG. 12. The irradiation process can again
take place with relative movement between the crown 126 and the
laser 150 being accomplished by rotation or other guided relative
movement between the two components. As the beam of the laser 150
passes out of the valve pockets 132 and into the region of the rim
130, the third mask 154 will harmlessly reflect the light energy
away from the rim surface 130, thereby protecting the already
reformed surfaces from further unwanted interaction with the high
energy laser beam 150. The third mask 154 further helps to avoid
excessive melting in the corners and along the edges of the valve
pockets 132.
[0039] Although many different types of lasers may be employed to
effectively accomplish the irradiating step of the subject
invention, a high powered direct diode laser has been found to
produce acceptable results.
[0040] It will be appreciated that the first 148, second 152, and
third 154 masks can be deployed in sequences other than those
described above. Furthermore, fewer than three or more than three
masks may be required during the irradiation step to effectively
reform the coating material as described herein. Furthermore, while
very specific coating materials have been proposed hereinabove for
use, these are not the only suitable materials. Rather, any
coatings suitable for use in a fusing operation using industrial
layers may be employed. For example, as is may be known from the
field of gas turbines, various common and proprietary powders may
be known to resist high temperature oxidation. Any such known
materials may be used, provided that the fused coatings conform to
the contours of the plume contact zone without cracking, corrosion,
or thermal oxidation. Another advantage of the fused piston crown
surface, according to this invention, results in the ability to
post-machine, if needed, the irradiated surfaces without chipping
or flaking away the sprayed coating. This subject method can be
accomplished in high production settings in a fast cycle time and
can be demonstrated to be repeatable and amenable to very precise
computerized control. The process is highly adaptable to in-line
production processes as well.
[0041] Obviously, many modifications and variations of the present
invention are possible in light of the above teachings. It is,
therefore, to be understood that within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described.
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